Hydrogenation of acetylene over low loaded Pd and Pd-Au/SiO2 catalysts

Acetylene hydrogenation and formation of surface deposits have been investigated on two series of Pd and Pd-Au/SiO2 catalysts differing in metal particle size (D=0.47 and 0.08). Gold was deposited via ionization of pre-adsorbed hydrogen over pre-reduced Pd/SiO2 in order to ensure selective poisoning of the Pd surface. The non-steady-state regime of operation and the accumulation of hydrocarbonaceous overlayer were tested in pulse-flow experiments. Concentration of surface hydrocarbonaceous deposits accumulated during different treatments was also determined by temperature programmed oxidation (TPO). Hydrocarbon overlayer was observed to form immediately and its presence appeared to be a necessary requisite to get steady-state conversion and selectivity data. A large excess of hydrogen suppressed the formation of carbonaceous laydown and increased the over-hydrogenation of acetylene. Presence of Au decreased the carbon coverage and improved the ethene selectivity. Decoration of Pd by Au and the morphology of particles explain the ethene selectivity improvement.